Chemistry

Chilli (Capsicum spp.)

Forget oranges and lemons… If you are looking for high doses of Vitamin C and other useful vitamins and minerals, together with antimicrobial and antifungal agents, the chilli fruit packs a punch. And one of its key ingredients is strong enough even to keep bears and elephants away.

1. In the main image for this article you can see one of the most common pieces of laboratory glassware, a beaker, filled with a selection of chillis bought in a UK supermarket. Next to it is a representation of the molecule capsaicin, which is one of the main components giving you a sensation of spicy heat when you eat a chilli. Interestingly, while mammals experience a strong, and sometimes even painful sensation of heat, birds are unlikely to feel the same, because their physiology is different and they cannot taste chilli heat. Different species of chilli, which have evolved in isolation in different regions and on different continents, contain mixtures of related molecules, so-called capsaicinoids, which all taste spicy to mammals. Their proportions help to determine whether a chilli fruit is merely spicy or blow-you-head-off hot, and you could refer to the Scoville scale to avoid surprises.

Not only do chillis spice up your food, they are also a veritable powerhouse of beneficial compounds which is high in Vitamin C, as well as A and B vitamins and useful trace elements crammed into these potent little packages. This makes them a useful food source for animals and it is thought that over time chillis have evolved to become hotter and so make them unpalatable to discourage mammals from eating them. Other than defending the fruit from greedy bears and so allowing birds to distribute seeds over a wider range, the molecules in the capsaicin family serve no useful purpose for the plant itself.

Chillis have become very useful to humans, reaching far beyond their uses in cookery. They can help to preserve food, as they have antifungal and antimicrobial properties. This is of particular interest in the hot and humid countries which often have the best spicy recipes, and there are hints that a taste for spicy food may have helped our ancestors to survive (The Quarterly Review of Biology , 1998, 73, 3-49).

But the uses of chilli fruit go beyond the kitchen: Some ancient civilisations used chilli extract as a weapon, and it has also long been used to “discourage” elephants away from farms and villages. More recently, capsaicin extract has been used in bear spray and in pepper spray, deployed for crowd control and self-defence. Several countries are developing highly concentrated chilli extracts as biological weapons, which can even be lethal if you have asthma or a weak heart.

Chillis can also be a force for good. Not only are they nutritionally valuable and contain anti-oxidants, capsaicinoids can also be used to numb pain and are used for osteoarthritis and other forms of long-term pain relief (Br. J. Anaesth., 1995, 75, 157 – 168). More recently, their ability to cause cell death has also been explored for certain cancer treatments.

No wonder, then, that the cultivation of strong chilli species is a promising new venture for farmers in many regions of the world.

Structures of vanillin and the most common capsaicinoids.

2. Capsaicin, dihydrocapsaicin and nordihydrocapsaicin are all shown in the image here and make up 98% of the capsaicinoids extracted from chilli fruits. As you can see, parts of their structures are very similar to vanillin, and this similarity is thought to determine their interaction with organisms. Vanillin is a substituted aromatic molecule with a hydroxy (OH) group, which allows it to interact with enzymes, but also makes it soluble in water. This functionality is preserved in the capsaicinoids, meaning they interact with similar enzymes, but the long hydrocarbon chains substantially alter their physical properties, making them insoluble in water. The hot flavour of chilli-based dishes lingers despite liberal application of water and beer, because these long chains in capsaicin do not dissolve well in water. And while you might not fancy it with a hot curry, a glass of milk or yogurt would help to wash the capsaicin away.

Even though the synthesis of capsaicin is straightforward from a chemical point of view, extraction from chilli fruits is still cheaper, safer and more widely accessible. Capsaicin is most commonly extracted from freeze-dried and crushed fruits with alcohols or hydrocarbon solvents such as hexane. It can also be extracted with near-critical carbon dioxide – you can find out more about using carbon dioxide for extraction in the context of rose oil. The same technique is also used to decaffeinate coffee beans.

Different analytical techniques are available to determine the capsaicin content of your extract. The traditional approach uses the Scoville test, which uses a group of human detectors to measure the heat of a solution after systematic dilution. Humans have a detection limit of 1 ppm, which is pretty impressive, but not as good as high-performance liquid chromatography (HPLC) with a detection limit of 0.2 ppm. HPLC works just like paper chromatography by exploiting the interaction of molecules with a support material. In the case of HPLC, this support is a column of solid material with a large surface area and different types of molecules adsorb with different strength to this solid, affecting how easy it is to wash them out with a solvent forced through the material at high pressure. The length of time it takes for molecules to pass through this column of solid material is measured as the retention time and it can be as accurate as a fingerprint for distinguishing between very similar molecules.

Capsaicin in a linear conformer together with a chilli fruit on a pair of safety specs

3. You might have noticed that the line-drawing of capsaicin looks quite different from the molecule shown in the main image at the top, while the conformation in the picture with the safety specs looks a lot more recognisable. The “curled up” conformer at the top is one of the lowest energy orientations calculated for this molecule and it is likely that this is stabilised by weak attractive interactions between the aromatic ring and the long chain of hydrocarbons, as well as the alkene group. There is also scope for weak hydrogen bonding both within and between capsaicin molecules. This preference for compact conformations, which maximise weak attractions (dispersion), happens for a lot of long-chain molecules, but may be less favourable in solution (the calculations to produce these structures were run on isolated molecules), where the molecules can also interact with solvent molecules. Similarly, the waxy nature of the solid arises from such weak interactions, which prevent the formation of an ordered, closely-packed crystalline lattice. In contrast, the smaller vanillin molecule both has polar groups and is quite small, allowing much more efficient interactions between individual molecules, as well as tighter hydrogen bonds to form.

As this little discourse suggests, being able to look at the structure of molecules is important for formulating predictions about their likely properties as well as their interactions with other molecules. While we often attempt to do this simply by inspection, it is also a key aim of computational studies of molecules, and recent studies of capsaicin and related compounds have used computation to investigate their ability as free radical scavengers (J. Phys. Chem. 2012, 116, 1200-1208) and predict their bioactivities (J. Agric. Food Chem. 2010, 58, 3342-3349).

As our comparison of vanillin and capsaicinoids suggested, interactions between molecules and enzymes can be preserved despite structural changes within families of molecules, and this has been exploited in the development of such compounds for medicinal uses. One of the main impediments in the testing of capsaicin on human subjects is that it is a strong irritant and in the search for alternatives, an extract from a sweet pepper led to the discovery and development of capsinoids, where the amide of capsaicinoids has been replaced by an ester. These capsinoids show similar pharmaceutical promise, but lack the spicy side-effects, and a synthetic route allowing their systematic screening has recently been described (J. Agric. Food Chem. 2010, 58, 3342-3349). Capsinoid chemistry and applications are less well-developed, but, by taking the heat out of the chilli, this family of versatile molecules may prove useful in novel ways once again.